Infrared or thermal imaging systems typically use a plurality of thermal sensors to detect infrared radiation and produce an image capable of being visualized by the human eye. Thermal imaging systems typically detect thermal radiance differences between various objects in a scene and display these differences in thermal radiance as a visual image of the scene. Thermal imaging systems are often used to detect fires, overheating machinery, planes, vehicles and people, and to control temperature sensitive industrial processes.
The basic components of a thermal imaging system generally include optics for collecting and focusing infrared radiation from a scene, an infrared detector having a plurality of thermal sensors for converting infrared radiation to an electrical signal, and electronics for amplifying and processing the electrical signal into a visual display or for storage in an appropriate medium. A chopper is often included in a thermal imaging system to modulate the infrared radiation and to produce a constant background radiance which provides a reference signal. The electronic processing portion of the thermal imagining system will subtract the reference signal from the total radiance signal to produce a signal with minimum background bias.
Thermal imaging systems may use a variety of infrared detectors. An infrared detector is a device that responds to electromagnetic radiation in the infrared spectrum. Infrared detectors are sometimes classified into two main categories as cooled and uncooled. A cooled infrared detector is an infrared detector that must be operated at cryogenic temperatures, such at the temperature of liquid nitrogen, to obtain the desired sensitivity to variations in infrared radiation. Cooled detectors typically employ thermal sensors having small bandgap semiconductors that generate a change in voltage due to photoelectron interaction. This latter effect is sometimes called the internal photoelectric effect.
Uncooled infrared detectors cannot make use of small bandgap semiconductors because dark current swamps any signal at room temperature. Consequently, uncooled detectors rely on other physical phenomenon and are less sensitive than cooled detectors. However, because uncooled detectors do not require the energy consumption of cooled detectors, they are the preferred choice for portable, low power, applications where the greater sensitivity of cooled detectors is not needed. In a typical uncooled thermal detector, infrared photons are absorbed and the resulting temperature difference of the absorbing element is detected. Thermal detectors include a pyroelectric detector, a thermocouple, or a bolometer.
An infrared window is a frequency region in the infrared spectrum where there is good transmission of electromagnetic radiation through the atmosphere. Typically, infrared detectors sense infrared radiation in the spectral bands from 3 to 5 microns (having an energy of 0.4 to 0.25 eV) and from 8 to 14 microns (having an energy of 0.16 to 0.09 eV). The 3 to 5 micron spectral band is generally termed the "near infrared band" while the 8 to 14 micron spectral band is termed the "far infrared band." Infrared radiation between the near and far infrared bands cannot normally be detected due to atmospheric absorption of the same.
Infrared radiation is generally focused onto a thermal detector by one or more infrared lenses. Infrared lens assemblies may be classified as zoom or single field of view and as wide or narrow field of view. A wide field of view infrared zoom lens may employ a fixed or a variable aperture stop. A fixed aperture stop allows a infrared zoom lens to maintain a constant F/Number, and thus a constant sensitivity, over the zoom range. Wide field of view infrared zoom lenses having a fixed aperture stop, however, are typically expensive to manufacture due to the number and size of lens elements and to the materials needed to construct the lens elements.